Technical
Definitions
Power:
The number of times a lens or lenses multiplies an image with respect
to the original size. Low power is recommended for scanning large
surfaces. Higher powers are generally used for inspecting small
areas.
Field
of View: The maximum area seen through a magnifier. The entire
field of view may not be in focus. See Flatness of Field.
Flatness
of Field: Due to the physical laws of optics, the outer part
of the image formed by a lens may be out of focus. The greater the
power and curvature of the lens, the more pronounced this problem
becomes. Magnifiers with multiple lenses can reduce this problem.
The visible area which appears in focus is the flat field.
Working
Distance: The distance from the magnifier lens to the object
being viewed. As magnification power increases, the working distance
decreases.
Aberration:
This is a distortion caused by the lens. Spherical aberration
is distortion of the image, as explained in Flatness of Field. Chromatic
aberration is distortion of colors being viewed, as explained in
Color Distortion.
Color
Distortion: Lenses produce a prism effect which causes the image
to develop false color fringes. This is due to the fact that different
colors focus at different points. Achromatic lenses correct this
problem by focusing many colors at the same point.
Parallax:
Parallax is the apparent misalignment of two different items
when viewed from an angle. It is the cause of improper registration
when stripping multiple color jobs. Parallax is eliminated when
viewing straight down on registration marks instead of from a slight
angle.
Coated
Lens: A coated lens helps to minimize or eliminate color distortion
caused by different colors focusing at different locations. Optimum
color viewing is obtained from coated achromatic lenses.
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